doc site cross references: call them pantsmark, pantsref instead of xmark, xref

<a xmark="foo"> looks like it could be some obscure HTML thing. Switch to <a pantsmark="foo"> which is more obviously a pants thing.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/41306457

Reviewed at https://rbcommons.com/s/twitter/r/1348/
Sync of https://github.com/pantsbuild/pants/commit/2d081c6911429c597bb4ca021704e4420c72ebd6